The logical conclusion based on upon the mindset I feel is healthiest (spoken about earlier) is simply to agree with her in the distrust of others. Everyone is seeking something from their interactions with us and we'd all be pretty stupid not to realise that. It might just be fun, but in many cases it's more. It's always important to bear in mind the motivations behind whatever people say. She has every right to distrust others.

The only reason it's a worry is because she doesn't feel reassured regarding it. That's because she doesn't feel like you understand where they're coming from. All you have to do is show that you understand the mindset by agreeing with her and feeding back the reasons for the mindset as I spoke of earlier. This doesn't remove the distrust of others, it does however remove the worry because you know your partner is aware of it. It shows they have the same social sense.

For me, I have added reassurance to people like this, I instantly lose attraction for anyone that tries anything with me while I'm in a relationship.(when they're aware of this). Obviously I make this clearly obvious as early as possible in most interactions.